Fix unittest key path in Brillo.

The unittests weren't able to find the keys if it's not run from the
update_engine_unittests directory.

Test: /data/nativetest/update_engine_unittests/update_engine_unittests
Bug: 26955860

Change-Id: I4c189cca2714986d3d2b2669b54a5dff9a488964
diff --git a/common/test_utils.h b/common/test_utils.h
index e4919d8..952cbea 100644
--- a/common/test_utils.h
+++ b/common/test_utils.h
@@ -190,6 +190,8 @@
 // Returns the path where the build artifacts are stored. This is the directory
 // where the unittest executable is being run from.
 base::FilePath GetBuildArtifactsPath();
+// Returns the path of the build artifact specified in |relative_path|.
+std::string GetBuildArtifactsPath(const std::string& relative_path);
 
 }  // namespace test_utils